Refiners are used for refining fibrous material. The refiner generally comprises refining members in the form of discs rotating in relation to each other and between which refining material passes from the inner periphery of the refining members where it is supplied, to the outer periphery of the refining members through a refining gap formed between the refining members. The refining material is then extracted at the outer periphery. One of the refining discs is often stationary while the other rotates. The refining discs are generally composed of segments provided with bars. The inner segments have a coarser pattern and the outer segments have a finer pattern in order to achieve fine refining of the refining material.
To obtain high quality refining material when refining fibrous material, the disturbances in operating conditions that, for various reasons, constantly occur must be corrected by constant adjustment of the various refining parameters to optimal values. This can be achieved, for instance, by altering the supply of water to produce a greater or lesser cooling effect, by altering the flow of refining material or adjusting the distance between the refining members, or a combination of these measures. To enable the necessary adjustments and corrections to be made, an accurate determination of the energy transmitted to the refining material is required, as well as of the distribution of the energy transmitted over the surface of the refining members.
To determine the energy/power transmitted to the refining material it is known to endeavour to measure the shearing forces that occur in the refining zone. What is known as a shearing force occurs when two surfaces move in relation to each other with a viscous liquid between the surfaces. Such shearing force is also created in a refiner when refining wood chips mixed with water. It can be imagined that the wood chips are both sheared and rolled between the refining discs, as well as collisions occurring between chips and bars. The shearing force depends, for instance, on the force bringing the discs together and on the friction coefficient. The normal force acting on the surface also varies with the radius.
In International Application No. WO 00/78458, a method and a measuring device are known for measuring stress forces in such refiners, the device comprising a force sensor that measures the stress force over a measuring surface constituting a part of a refining disc, and in which the measuring surface comprises at least parts of more than one bar and is resiliently arranged in the surface of the refining disc. However, it has been found that this measuring device is very sensitive to temperature fluctuations, which are usual in the applications under discussion, and it therefore often gives incorrect values for the force, which cannot be used to control the refining process, for instance. Furthermore, a value is obtained only for the shearing force, in one direction. A method and a measuring device for measuring stress forces in such refiners are also known, as set forth in International Application No. PCT/SE02/01501. A solution is proposed therein to the problem of sensitivity to temperature fluctuations, as well as measurement of the shearing force in two directions.
However, neither of these publications proposes any solution to the drawback of other forces influencing the refining segments, such as the normal forces, not being taken into consideration.
One object of the present invention is to solve the problems mentioned above and thus provide a method and a measuring device that enables a more complete and correct result than previously known devices, and that provides more information concerning the actual refining process.